Does your mother-in-law (daughter-in-law) drive you crazy?There’s a legit scientific reason why you and she don’t get along.

There’s a scientific reason why you and your mother-in-law don’t get along. (iStock)

By Sheril KirshenbaumBy Sheril KirshenbaumNovember 12, 2016

A dear friend of mine dreads the annual Thanksgiving pilgrimage to her mother-in-law’s house. She loathes everything from the five-hour car ride to the over-the-top seasonal decor.

“I can’t turn around without knocking some gourd off its perch,” she says. But her biggest grievance is the decade of unsolicited, awkward — and, she feels, often inappropriate — advice that comes with the visit.

During her pregnancy, my friend’s mother-in-law cautioned her not to neglect her husband’s “emotional and sexual needs.” When she was breast-feeding a new baby, her mother-in-law said she looked like “a human pacifier.” And at my friend’s 10th-anniverary party, her mother-in-law inquired whether “everything was still hunky-dory” in the marital bed.

Friction between mother- and daughter-in-law isn’t unique to my friend. According to Terri Apter, a psychologist at Britain’s University of Cambridge who has written several books on family relationships, in-laws rank among the top three reasons for marital strife, along with infidelity and stepchildren. About 60 percent of married women (but only 15 percent of married men) say they experience sustained stress because of their spouse’s mother.

Science can provide some insight into what’s going on.

From an evolutionary perspective, a strategy called kin selection — that is, promoting an individual’s own genetic lineage by favoring blood relatives — serves an important purpose. We’re most kind and generous to close family members, and we’re often suspicious of those without a genetic link who have influence over our children and grandchildren. As a result, mothers- and daughters-in-law have been gently nudged apart for hundreds of thousands of years.

A social norm of male-dominated chimpanzee societies suggests our modern-day conflicts may have ancient roots. According to Duke primatologist Vanessa Woods, unrelated female chimps within a group frequently act viciously toward each other.

While males stay with their mothers, sexually mature female chimps leave for other communities where “mothers-in-law” (blood relatives of males she will presumably mate with) are waiting to abuse them, Woods says. The newcomers are relegated to the bottom of the social hierarchy and suffer frequent beatings.

Yet our equally close cousins the bonobos behave in the opposite way. As with chimps, sexually mature females emigrate into new communities, but bonobo families are all about girl power. Females form alliances, dominating the males, and newly arrived females are welcomed. They are groomed and adored as they establish strong bonds with the other females in the family.

As our two primate cousins suggest, the dynamics of mothers- and daughters-in-law are often complicated, but scientists think they may have also had a surprising effect: the evolution of menopause. While many animals experience declining fertility as they age, just three species stop reproducing long before the end of their lives: humans, killer whales and pilot whales. A study led by biologist Mirkka Lahdenpera suggests that menopause may have evolved in humans so that mothers- and daughters-in-law would be less likely to be pregnant at the same time.

By looking at 200 years of data, much of it from pre-industrial Finns, researchers found that the child of a young mother was 66 percent less likely to survive when that mother gave birth around the same time as her mother-in-law.

The theory goes that menopause arose because it was better for everyone when Grandma pitched in to help raise a child rather than add another who would compete for resources and attention. While the evolutionary forces at work acted on the entire human population, it can be argued that the loss of fertility, hot flashes and weight gain are fair justification for every mother-in-law to hold a unconscious Darwinian grudge.

Of course, not all relationships are strained. Many of my girlfriends get slightly nauseated when I talk about my husband’s mother, Susan, because I can’t help but enjoy spending time with her. She’s thoughtful, a master chef and always willing to help tidy up the house when she visits. I’ll never cook at her level or be as well organized, and we’re both okay with that. It also probably doesn’t hurt that we live more than 2,000 miles apart.

Still, despite our warm rapport, the data isn’t exactly on our side: Research by sociologist Terri Orbuch showed that women who reported a good relationship with their husband’s parents after a year of marriage were 20 percent more likely than others to get divorced. Orbuch suspects these wives may not have set appropriate emotional boundaries with in-laws early on. (It can be a strain on a marriage when no topic is off limits.) If you’re wondering, the opposite appears to be true for men. Husbands were 20 percent less likely to get divorced if they described a close relationship with their in-laws early in marriage.

Regardless of these figures, if our genes push unrelated family members apart to promote the success of genetically related members, the culture we live in influences relationships. Bonobo behavior, for instance, suggests that when female in-laws aren’t competing for male attention or approval, they can establish stronger alliances among themselves, Woods says. To extend this to humans, women might achieve more harmony by focusing on building connections with each other rather than by fixating on the men in our family circle.

On an individual basis, neuroscience offers hope that even the most testy mother-and-daughter-in-law relationships may be salvaged over time.

Studies have show that new experiences reshape areas of the brain involved in emotion in small ways as we store and process new information. This brain plasticity means that the way we feel about others — even about our in-laws — can change as we develop a deeper understanding of their motivations and values.

Last Christmas, something changed for my friend and her perception of her mother-in-law. Her children were finally old enough to grasp all that Grandma had done to make the holidays they spend together special, and as the kids beamed while taking in an abundance of lights and music, my friend saw the holidays through their eyes. She finally understood how hard her mother-in-law was working to create a sense of wonder for her family, and that suddenly mattered more than the occasional passive-aggressive or inappropriate remark directed at her. Before leaving, she embraced her husband’s mother with a sincerity she never had before.

“I actually felt grateful to have her in our lives,” she told me while packing for this year’s visit. “And most of all,” she wryly added, “I’m grateful she’ll be the only mother-in-law I’ll ever have to put up with.”